1. Field of the Invention
The present invention relates to a method of producing an ultra-thin copper foil with a carrier and an ultra-thin copper foil with a carrier produced by the same, in particular, to an ultra-thin copper foil with a carrier suitable for a printed circuit board for high density ultra-fine circuit applications (fine pattern), a multilayer printed circuit board and a chip on film circuit board.
2. Description of the Related Art
A printed circuit board is produced as follows.
First, a thin foil for forming a surface circuit is put on an electrically insulating board comprised of a glass-epoxy resin, a glass polyimide resin, etc., and then they are heated and pressed to produce a copper-cladding laminate board.
Next, this copper-cladding laminate board is successively formed with the through holes and plated at the through holes, then the copper foil at the surface of the copper-cladding laminate board is etched to form circuit patterns of the desired line widths and pitches of adjacent lines. Finally, a solder resist is formed and other finishing works are performed.
The copper foil used for the copper-cladding laminate board is roughened at the side to be hot-bonded to the base board, exhibits an anchoring effect on the boards by the roughened surface, to improve the peel strength between the base board and the copper foil to thereby ensure reliability on the printed circuit board. Further, recently, the roughened surface of the copper foil is covered with a resin for bonding such as an epoxy resin, and the copper foil with resin that this resin for bonding is made to an insulating resin layer in semi-cured state (B stage) is used as a copper foil for forming a surface circuit, then a printed circuit board, in particular a build up circuit board is produced by hot-bonding the side of the insulating resin layer to the substrate. A “build-up circuit board” is a type of a multilayer circuit board and a printed circuit board obtained by forming an insulating layer and circuit patterns one by one in turn on an insulating board, plating holes (via holes) formed by a laser method or photolithography, and stacking the circuit layers while connecting the layers conductively.
This circuit board can handle the increasingly higher densities of various electronic parts. By making the via holes increasingly small, the circuit patterns can also made higher in density. Therefore, there is a demand for a printed circuit board with circuit patterns enabling a circuit of fine line widths and pitches of adjacent lines, that is, fine patterns. For example, in the case of a printed a board having a high density ultra-fine circuit of line widths and pitches of adjacent lines of around 30 μm has been demanded.
If a thick copper foil as the copper foil for forming such a fine printed circuit board is used, the etching time until reaching the surface of the board becomes longer. As a result, the verticality of the sidewalls of the circuit patterns formed is ruined, and it may lead to disconnection when the circuit line width of the circuit pattern to be formed is narrow. Therefore, as the copper foil used for fine pattern applications, copper foil having a thickness of less than 9 μm is being required.
However, such a thin copper foil (hereinafter, it may be called an ultra-thin copper foil) has weak mechanical strength, is easily wrinkled and creased, and the foil sometimes goes out when producing a printed circuit board, so as an ultra-thin copper foil used for fine pattern applications, an ultra-thin copper foil layer on one surface of a carrier copper foil through a peeling layer is being used.
Now, an ultra-thin copper foil with a carrier which has the thickness of about 5 μm is mostly used, and furthermore it is required for a thinner foil.
By the way, as for a printed circuit board, density growth is progressing in the IC mounting board which drives the liquid crystal display which is a display part, such as a personal computer, a cellular phone, a plasma display unit, etc. The IC mounting board is also called as a chip on film (COF), since the IC is directly mounted on the board film.Spec pages at page 26
In COF mounting, the position of the IC is detected by the light passing through the film where a copper foil is etched away for forming circuit patterns. However, the visibility (capability of detecting The IC position by light) of the film that used a conventional ultra-thin copper foil with a carrier is not so good. The cause is that the surface roughness of an ultra-thin copper foil with a carrier is large. The film part through which light passes is a part that is removed by etching of the unnecessary part of a copper foil other than the circuit part formed by a copper foil. When the copper foil is bonded on the film, the unevenness of the surface of the copper foil is printed on the surface of the film, and then the unevenness of the surface of film enlarges. When light passes, because of the unevenness a quantity of light going straight ahead lessens. Therefore visibility gets bad. That is to say, it gets bad visibility that the roughness of the surface of the bonding copper is large.
An ultra-thin copper foil with a carrier is a thing that a peeling layer and an electroplated copper layer are formed in this order on the one side of a carrier foil. And the outermost surface of this electroplated copper layer is made as a roughened surface. Although the electroplated copper layer is used as the copper foil for a circuit baseboard, the condition of the surface roughness of the carrier foil has a very large impact on the number of pinholes of the electroplated copper layer or visibility in COF. Therefore, in a case of using an electrodeposited copper foil as a carrier foil, by using a shiny side that roughness is small (the side of an electrodepositing drum), historically the number of pinholes is decreased as much as possible and surface roughness is reduced (refer to Japanese unexamined patent publication No.2000-269637). However the control of the surface roughness has been performed by controlling the surface of the electrodepositing drum because roughness of the shiny side of the electrodeposited copper foil depends on surface roughness of the electrodepositing drum, there is a ceiling to control level, and administrative and maintenance expense increases to control it. Therefore it is expensive, the present condition is being unable to spend such expense in a usual product.
Moreover, when a rolled material is used as a carrier foil, because the surface roughness of a rolled material is smaller than an electrodeposited copper foil, a rolled material is more excellent in visibility than a case that an electrodeposited copper foil is used as a carrier foil. However, it was very difficult to produce a carrier foil that satiate user's specification, because the current cost of a rolled material is more expensive than an electrodeposited copper foil, and because it will be occur that rolling lines remain in the surface, etc.
Already mentioned, it resolved that the number of pinholes and the surface roughness (visibility of film) of an ultra-thin copper foil with a carrier depend on the surface roughness of the carrier foil, and the dependence become larger, as thickness of an ultra-thin copper foil is thinner. However, it is the present condition that the method of producing the ultra-thin copper foil with a carrier, without changing the current cost of production manufacturing, and using a carrier foil which has such a small surface roughness that satisfies user's way, is not developed.